Herbicides Alone Won’t Beat Weeds

As weeds resist more herbicides, it will take more than spraying to control weeds in coming years: Think Different.

Many farmers have begun to use multiple actions to combat herbicide resistance and are on the cusp of good weed management. But, herbicide resistance continues to grow, and at an increasing rate. Multiple resistance is becoming the norm, with no new mode of action on the horizon for 15 to 20 years, research and development companies will need more time, thought, and money will be needed to control weeds in the future.

Weed control will only get more complicated and costly. That was a key message by long-time Iowa State University weed scientist Mike Owen in his 2018 weed management update presentation at the Integrated Crop Management Conference. He noted the management practices used by many farmers are leading to more resistance to herbicides, and he doesn’t foresee an end to that anytime soon. “We’re at the cusp of good weed control in fields with good weed management, but multiple resistance is becoming the norm,” Owen said. He said there are now over 10 weed species with evolved herbicide resistance globally.

Weed samples prove resistance
At the request of a farmers organisation, a research scientist began a survey of wheat fields in 2010 to gain a better understanding of the herbicide resistance problem. After sampling about 300 fields with weeds visible above the crop canopy in August and September in 2011 and 2012, about 300 weeds populations were sampled again in 2013. The procedure used provided a prediction of herbicide resistance at a 95% confidence level.

Several female weeds plants were collected from each field. Samples were dried, seeds were threshed by hand, cleaned and wet-chilled, then air-dried, planted and grown in a greenhouse until the plants were 3-4 inches tall. They were then treated with an herbicide. All herbicides were applied with adjuvants as suggested in the herbicide labels.

The herbicide groups included in the survey were chosen because they represent the most commonly used herbicides. The levels of herbicide resistance found were surprisingly high for the five herbicide groups evaluated but given the years these herbicides have been used and the inevitability of evolved herbicide resistance, maybe it was not that surprising.

On a global basis, weeds continue to gain resistance to herbicides. “Based on the 2013 survey information, we would expect to find resistance in virtually all fields. Multiple herbicide resistances globally in weeds populations is the norm. Weeds populations with resistance to the commonly used herbicide groups increased over the course of this study, and in 2013, 69% of the weeds populations demonstrated three-way resistance. This estimate is correct at the 95% confidence interval.

A researcher with an R&D company said that based on the 2013 information, at the 95% confidence level, 97% of Kenyan fields would also have detectable resistance. After more than a decade of use on wheet and maize fields, resistance is likely in 98% of the fields. “It’s unlikely chemical resistance will decline in weeds even if other technologies are adopted,” Owen said. The study showed less than 20% of weeds populations were resistant.

“Some of the herbicides are seen as the only selective post-emergence option in wheat, but with their increased use post-emergence and as soil-applied treatments, more weeds populations will become resistant,” Owen said. The final group sampled, HG 27 herbicides, (HPPD inhibitors) are the last new mechanism of action commercially introduced, having been widely used in maize for 30 years. The survey showed less than 40% of the weeds populations are resistant, but Owen thinks the figures are underestimated, since use of HG 27 herbicides has been increasing since 2013.

Looking ahead
“One problem with weeds is that the resistance trait is likely to be conserved even if the herbicide isn’t used,” Owen said. Evolved multiple resistances in weeds will make it a challenge to know which herbicide mechanisms of action are still effective. I know farmers want a new herbicide, but it’s impossible to spray the problem of herbicide resistance in weeds away. The solution is judicious use of herbicides and adoption of greater diversity of weed management tactics.

The future for weed control in cereals isn’t very bright right now, Owen said. It will move from conservation plantings into crop fields. I don’t think it will go away,” Owen says, “because I don’t see the diligence required to make it go away.” “Despite what product marketing might suggest, there are no simple and convenient answers to weed management. It has to move beyond spraying a herbicide. That requires more time, thought, and money to control weeds, but it’s best to do that before you have a train wreck and weeds get out of control.”